This is an A-1 revision of a competing continuation application for DK55023 which has been active for the past 10 years, and which has served as the basis for the PI's exploration of growth factor, signaling and cell cycle control in the pancreatic beta cell. It is now clear that both Type 1 as well as Type 2 diabetes result in part or entirely from beta cell deficiency and/or dysfunction, and that beta cell replacement can reverse diabetes in humans. The two major hurdles to beta cell replacement and regeneration are now an incomplete ability to prevent auto- and allo-immune attack, and a grossly inadequate supply of human beta cells. This renewal application addresses the second hurdle. While it is now abundantly clear that it is possible to activate rodent beta cell replication, human beta cells remain refractory to replication induced by growth factors, nutrients, and signaling pathway activation. Our group has characterized the murine and human beta cell cycle control machinery and has developed cell-cycle based approaches to driving robust human beta cell replication. In this application, we describe studies focused on three interesting and surprising cell cycle regulators that have emerged from prior versions of this grant: p57, cMyc and p107. Most investigators would not intuitively have selected these three as likely candidates for normal cell cycle control and human beta cell expansion. We believe that the data presented herein move them from among the least interesting to among the most interesting and potentially important candidates for regulating beta cell proliferation in rodents and humans. Thus, the Specific Aims of this proposal are: Specific Aim 1. To Define the Importance of p57 in Restraining Rodent and Human Beta Cell Replication. Specific Aim 2. To Define the Pathophysiology and Therapeutic Potential of Mild, Graded and Transient cMyc Overexpression in Rat and Human Beta Cells. Specific Aim 3. To Define the Relevance of the Pocket Protein, p107, in Mouse and Human Beta Cell Cycle Repression. These studies should provide important insight into how beta cell replication is controlled and lead to insight into how one might develop methods to activate these key molecules in the human beta cell, and permit inducible, regulated human beta cell replication.